http: fix build warning under LLVM

When building the code using LLVM Clang without NGHTTP2, I was getting
this warning:
../lib/http.h:155:1: warning: empty struct is a GNU extension [-Wgnu]
Placing a dummy variable into the data structure silenced the warning.

lib/http.h

@@ -151,6 +151,8 @@ struct HTTP {
 struct http_conn {
 #ifdef USE_NGHTTP2
   nghttp2_session *h2;
+#else
+  int unused; /* prevent a compiler warning */
 #endif
 };